Magical powers are not contingent upon one's virginity or lack thereof.
Enforced: The author thinks virgins are better, and gives them special powers to show that.
Lampshaded: "Great supernatural powers....this means I'll die a virgin".
Invoked: Virginia already was a Celibate Heroine (or asexual) before acquiring her power. The person/entity that granted her the ability knew it would be easier for a person devoid of romantic/sexual interest to maintain this power.
Virginia (or someone close to her, like her father) learns about the existence of the Virgin Power, so she's infused with it in order to protect her virginity.
Draco, an Empire's double agent and consummate manipulative bastard learns about the virgin-clause on Virginia's power, so he decides to charm her in order to fall in love with him and eventually take away her gift.
Defied: Virginia doesn't want her powers anymore, and screws Bob the first chance she gets.
Discussed: "What makes virgins so special, anyway?"
Conversed: "I dunno...maybe it's their rarity. Or their perceived purity. You know, like a diamond, or something".
Implied: Seamus' sex life was ambiguous, but his hobbies, as well as his powers, weren't.
Virgin Tension makes Virginia's love life awkward. She also can't help but feel like she's beingused... She's also constantly fighting off villains who try to seduce her so she'll lose her powers.
Seamus gains powers from his virginity... But because he's a bitter loner, he gains dark powers. And his unicorn is not a pretty little pony, but an ebon steed with glowing red eyes and a jagged horn. He's become the living embodiment of Pure Is Not Good
With such a powerful incentive for chastity, birth rates are dangerously low.
Virginia decides that her powers are incredibly awesome, and that being a Celibate Heroine is a small price to pay for them. She accepts her role as Barrier Maiden. And she's a more well-rounded person because she's focused on things other than romance.
Seamus' dark powers are just Rule of Cool and he is admired by many people for not following people's standards.
Seamus was pressured for not being a virgin, despite his powers.
Draco learns that Virginia's powers are dependent of her virginity, and rapes her when he fails to seduce her. Virginia not only loses her powers, but is also psychologically broken and gets PTSD as a result of the ordeal. She ends up resenting the powers she once had, as well a the goddess that gave them to her.